Lichen News

November 2012: Chalara Ash Dieback & Lichens

The news that Chalara Ash Dieback (Hymenoscyphus pseudoalbidus) has reached Britain is deeply depressing. The following are links to useful information on the disease.

Forestry Commission Questions and Answers

Chalara dieback - Key scientific facts

Guardian article by Patrick Barkham

British Lichen Society Statement & The Importance of Ash Trees to Lichens

Likely impacts of Chalara Ash Dieback on lichens in Breconshire and Radnorshire

Woodland Trust

Current research and European initiatives

Swedish paper indicating the potential for resistance to dieback

Flora Locale Ash dieback, tree planting and the plant trade

Guest blog on Mark Avery's website, by Peter Marren

It looks like the impact will be very severe for lichens that depend on old Ash trees. This disease has the potential to have an even more serious than the impact on lichen diversity in Britain than Dutch elm disease. Reports indicate that samplings and young trees are rapidly killed but that older trees hang on for quite some time. The effect on older trees sounds like perpetual annual pollarding and death of old trees so far seems to have mainly been by secondary infection, especially honey fungus. At best old trees will have an much increased death rate with limited opportunities for replacement regeneration to occur. There are suggestions of 1% or 2% resistance to the disease. If confirmed, this is much better than for Elm and bodes well for the eventual recovery of Ash. In the mean time, however, this death rate has the potential to remove well over 90% of older lichen rich Ash trees in a few decades. It will take at least 150 years for the old Ash tree habitat to start developing again significantly and over 250 years for a full range of old Ash habitats to be restored.

Some photographs of Britain & Ireland's heritage of old Ash and lichens

Ash & Lichens

Ash is an important lichen substrate, but nationally there are few specialists that are wholly or largely confined to it, perhaps less than there was on Elm. There are, however, some very significant species; in our area Wadeana dendrographa (NT, BAP, IR & NS), a specialist of senescent Ash, found very rarely on Oak, still has important populations in Dorset and the New Forest. Ash also provides a refuge for former Elm specialists such as Caloplaca virescens (EN, BAP& NS but probably now NR), not seen for a bit but still known on a few Ash trees in Hampshire by Francis Rose, Anaptychia ciliaris subsp. ciliaris (EN, BAP & NS) and Cryptolechia carneolutea (EN, BAP & NS). The particular importance of Ash, however, is in providing a good substrate for more base demanding generalist lichens of old trees. This importance is enhanced by the fact that Ash is very widespread, it matures earlier than Oak and, given specific woodland histories and ecological circumstances, can provide the only host for significant assemblages of rare and threatened species in some sites. This latter feature can extend regionally, for example Ramonia nigra (CR, BAP, NR) has been found inside hollow Beeches, Hollies and one Ash and Oak bark in the New Forest but is only known from inside hollow Ash trees in the rest of England and in Ireland. Ash is often a particular good habitat for the internationally threatened Lobarion community, for which Britain has a special European responsibility. In mixed woodland with a good range of species in areas of low air pollution, most species found on Ash will also be found on other tree species. For example to the south, base rich Oaks will also support a similar range of leafy species along with crust forming specialists species that are less common on Ash than Oak. To the north and west, the Lobarion can be equally well developed on Hazel and Sallow. In such woods we are looking serious population reductions but, it is to be hoped, few actual immediate losses of species. Some losses, however, will probably probably occur even in woods where several species of trees with base rich bark are widespread. Even this best case scenario, of serious declines and some species loses, will be a very serious threat to the longterm survival for many of the lichen species for which Britain has an international responsibility. This Swedish paper link suggest this might be to optimistic.

Spye Park

There are much more gloomy prospects in many circumstances, particularly where lichen populations have already been stressed by air pollution and Ash has acted a refuge from acidification. Ash has also been a final refuge for some Elm specialists, as discussed above, and some species have only survived the death of the Elms because of small populations surviving on Ash. In our area, Spye Park in Wiltshire has a notable assemblage of former Elm species such as Bacidia incompta (VU BAP & probably now NS with the loss of Elms) and Gyalecta flotowii (NT & NS). One Ash also has Lobaria pulmonaria (IR) in what is now the most eastern surviving colony in Wiltshire, now that the population on Oak in Savernake Forest has been lost. This park has different surviving lichen assemblages of importance on dry bark and dead wood on old Oaks. However, the Oaks here have been too acidified by past air pollution to support Lobarion species and Oak rarely supports former Elm specialists. The park has already suffered serious losses of diversity with the past loss of Elms, including the loss of Collema fragrans (EN, NR, BAP, IR) and Anaptychia ciliaris subsp. ciliaris (EN, BAP & NS). The loss of old Ash will devastate the lichen diversity here.

Pictures from Spye Park

Cranborne Chase

Also very threatened are the woods of the Rushmore Estate on Cranborne Chase. Here numerous old Ash trees supports one of the richest Lobarion assemblages in southern England. For historic management reasons there are few old Oak and much of the interest of this site depends on Ash trees.

Pictures from Rushmore Estate

Savernake Forest & New Forest

Savernake Forest is very different; Ash appears not to have historically been frequent, even on the more base rich soils. It was probably browsed out when the forest was a heavily grazed deer park. The lichen interest here is mainly found on the old Oaks and some ancient Maples and Beeches. Ash has, however, widely recolonised on chalky soils and did provide a potentially very promising future lichen substrate that would have helped bridge a large generation gap in the Oaks. Now it will not. In the New Forest Ash survived the decline in tree diversity caused by the navy fellings of the 17th and 18th centuries as scattered old pollards. Felling in the presence of grazing caused serious declines in Ash, Small Leaved Lime and Hazel, which had survived centuries of grazing previously. Unlike the other two species, however, Ash has recovered considerably on the most fertile soils, especially in the internationally important old growth flood plain woodlands, an Annex 1 Priority Habitat in the Habitats Directive. The oldest Ash support important lichen species such as Wadeana dendrographa (NT, BAP, IR & NS), confined to Ash here, and are important refuge trees for declining Lobarion species. The numerous younger Ash represented a promising future resource of base rich bark for colonisation by rare lichens. Now the New Forest looks to be put back to a condition worse than produced by the navy fellings as far as Ash is concerned,

Conservation Status: CR = Critically Endangered Red Data Book species, EN = Endangered Red Data Book species, VU = Vulnerable Red Data Book species, NT = Near Threatened Red Data Book species, BAP = Biodiversity Action Plan Priority Species, IR = International Responsibly species, NR = Nationally Rare & NS = Nationally Scarce.

Pictures from the New Forest

What Can Lichenologists Do?

Short of a miracle cure there is probably not much that can be done for crust forming species growing exclusively or largely on Ash. Rare leafy species may potentially be translocated to suitable alternative tree species, but this is not an easy procedure to get right, nor might suitable trees be present.

What can be done is analyse the likely impact. Where there are detailed survey reports, SSSI site dossiers and BAP species dossiers this is possible. Information is certainly not comprehensive and more survey will certain be need to fully assess the likely impact.

In many cases probably nothing much can be done. In other areas some actions might help. Old growth lichen populations are already under stress from air pollution and increasing shade within traditional pasture woodlands. The pollution includes both short range ammonia pollution from agriculture and apparent long range low level or residual effects from declining acidifying pollution. Reducing these negative pressures would ensure that declining lichen species were more dynamic and more able to colonise other tree species. In much of England and Wales, residual acidification and or very low but significant levels of sulphur dioxide appear to be constraining the ability of Lobarion community species to colonise, although mature thalli are still growing well and suitable non acidic uncolonised trees are present. In these areas translocation of thalli might be essential if much of the Lobarion community is to survive. Even in clean air areas declining grazing pressure in pasture woodlands is increasing shade to levels that are shading out rare lichens all together. Sustainable grazing regimes for upland woodlands that allow both some regeneration and maintain open conditions are desperately required. Far too many traditionally grazed lichen rich woods are still fenced off from all grazing, an action known to remove much of the lichen interest of previously grazed woods.

The closest ecological approximations to Ash are two European trees that did not get to Britain this interglacial; Sycamore and Norway Maple. Both clearly thrive in the current climate and their absence from Britain is presumably a chance result of the vagaries of postglacial dispersal. They also represent a missing niche in British forests of canopy maples. Changing prevailing conservation attitudes to accept these tree species as honouree natives would provide potential bridging habitats until resistant Ash populations develop. At least where maturing and maturing trees are present and have not already been felled by conservationists.

Finally lobby for changes to the international trade in plants. Currently it there is no effective action to prevent repeated introductions of deadly plant diseases. If this goes on we will have no old trees left.

Neil A Sanderson 12/11/2012, updated 14/11/2012. Additional link added 29/11/2012